It has been known for many years how to hydrolyze and dissolve organic matter from wood. Generally, there are two ways to achieve hydrolysis and dissolution of the carbohydrate substances in wood; the use of dilute acids at high temperature and pressure or the use of concentrated acid at low temperature and pressure. In the Bergius-Rheinau process wood is shredded to chips which are treated with concentrated hydrochloric acid. During the treatment two-thirds of the wood is dissolved by the acid in the form of mono- and oligosaccharides, one third remains as lignin. The saccharides are separated from the hydrochloric acid solution in the next phase. The separation is achieved by evaporation in a tubular evaporator under vacuum at a temperature of about 36° C. After evaporation the resulting syrup that contains 55 to 65% wt of saccharides is spray dried. The resulting solid product contains 1 to 2% wt of hydrochloric acid, some 8% wt of water and about 90% wt of saccharides (cf. F. Bergius, Current Science, 1937, 632-637). Hence, the solid was not very pure. Moreover, as indicated in U.S. Pat. No. 2,944,923, the evaporation to remove the water requires a large heat supply. In order to overcome these drawbacks, it is proposed in U.S. Pat. No. 2,944,923 to pass the saccharides solution from the hydrolysis of wood in a single passage at atmospheric pressure upwardly through externally heated evaporator tubes in which it partially evaporates. During the evaporation especially the hydrochloric acid is removed. The temperature in the evaporator tubes is about 140 to 150° C. The product of such a process is a more concentrated saccharides solution. From the examples it appeared that the solution still contained about 7% of all hydrochloric acid that was introduced into the evaporator. Further, although it is mentioned in U.S. Pat. No. 2,944,923 that the process incurs hardly any saccharide loss, it is well known that at such temperatures carbohydrates decompose, especially in an acidic aqueous environment.
A different way of removing the volatile hydrochloric acid from a sugar-containing wood hydrolysate is described in DE 362230. According to the therein described process a hot liquid heat carrier that is immiscible with the aqueous hydrolysate is mixed with the aqueous hydrolysate. Examples of heat carriers are shale oil, refined petroleum and refined paraffin oil. The heated carrier liquid drives the volatile hydrochloric acid out of the sugar-containing wood hydrolysate. Since the heat carrier has been heated the temperature of the resulting mixture is increased and hydrochloric acid and some water are vaporized. However, it has also been stated in DE 362230 that for the hourly treatment of 100 liters of aqueous hydrolysate 10,000 liters of recycled heat carrier is required. Moreover, the heat carrier is stated to absorb some water and hydrochloric acid. It is therefore proposed in DE 362230 that in order to render the heat carrier suitable for recycling after separation from the hydrolysate, it has to be treated over calcium chloride in order to remove any absorbed hydrochloric acid and water. The inventory of heat carrier in this process is therefore huge. Moreover, the product of this known process is not the solid saccharide but an aqueous saccharide solution.
Chinese patent application CN1936025 also describes the production of a liquid polydextrose solution wherein the production may involve a variety of treatments of a crude polydextrose solution. Such treatments include reverse osmosis, ion exchange treatments, filtration and discoloration with active carbon. In an embodiment a solution of glucose is treated in a triple-effect evaporator into which an oil is introduced as an indirect heat transfer medium. The result is a more concentrated glucose solution.
Solid saccharides are produced in the process according to US 2013/0168226. In this process a container is filled with a hot heat carrier which is immiscible with an aqueous hydrolysate of biomass. Hydrolysate, comprising water, hydrochloric acid and saccharides, is sprayed into the hot heat carrier, so that evaporation of water and hydrochloric acid takes place. Since the hydrolysate has to be sprayed just under the surface of the heat carrier, a nozzle manifold is required to ensure a good distribution of hydrolysate over the cross-sectional area of the container. Saccharides precipitate in the form of solid particles and these particles are recovered from the hot carrier. This runs the risk that the bottom is contaminated with scale which complicates the recovery of the sacharides precipitate. Although this process yields solid saccharides, it is evident that the temperature of the heat carrier cannot be very high in order to avoid decomposition of saccharides. Therefore, the amount of heat carrier must be very high in order to provide sufficient energy for the evaporation of hydrochloric acid and water. The process according to US 2013/0168226 suffers the same drawback as the process according to DE 362230, viz. in that it requires a large amount of heat carrier to cause sufficient evaporation of hydrochloric acid and water.